Ch# 11. Rolling Contact Bearings 28/06/1438. Rolling Contact Bearings. Bearing specialist consider matters such as

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1 Ch# 11 Rolling Contact Bearings The terms rolling-contact bearings, antifriction bearings, and rolling bearings are all used to describe the class of bearing in which the main load is transferred through elements in rolling contact rather than in sliding contact In a rolling bearing the starting friction is about twice the running friction, but still it is negligible in comparison with the starting friction of a sleeve bearing Load, speed, and the operating viscosity of the lubricant do affect the frictional characteristics of a rolling bearing It is probably a mistake to describe a rolling bearing as antifriction but the term is generally used throughout the industry Rolling Contact Bearings Bearing specialist consider matters such as Fatigue loading Friction Heat Corrosion resistance Kinematic problems Material properties Lubrication Machining tolerances Assembly and cost 1

2 Rolling Contact Bearings A device to permit constrained relative motion between two parts. The bearings that support the shafts of motors and pulleys are subject to a radial load The bearings in a car wheel are subject to both thrust and radial loads. 2

3 The bearings in this stool are subject to a thrust load Roller Thrust Bearing Ball Thrust Bearing 11-1 Bearing Types Anti-friction bearings are manufactured to take 1. Pure radial load 2. Pure thrust loads, or 3. A combination of the two Sliding contact, rolling contact Roller bearings Ball bearings Journal Bearing 3

4 11-1 Bearing Types Nomenclature Single row (Ball) Four Essential Parts of Ball Bearings 1. Outer Ring 2. The Inner Ring 3. Ball or Rolling Element 4. Separator Low cost bearings have no separator, It has very important Function (rubbing contact will not occur) 11-1 Bearing Types Nomenclature Double row (Roller) 4

5 11-1 Bearing Types 1. Deep groove 2. Filling notch 3. Angular contact 4. Shielded 5. Sealed 6. Self-aligning 7. Thrust 8. Self-aligning thrust 9. Double row Conrad or Deep Groove The balls rotate in a deep groove machined into the inner and outer races. A cage maintains Ball space. This type of bearing exhibits a good radial load capacity, a fair axial load capacity and a fair capability to resist misalignment. 5

6 Maximum Capacity (filling Notch) Maximum capacity bearings have a higher load capacity than the Conrad type but diminished thrust capacity. These bearings have no cage to separate the balls. A filling slot is used to load as many balls as possible. Angular contact Bearings 6

7 Sealed Bearings Sealed Angular Seals and shields provide protection from contamination and as a retainer for lubricant. Seals provide better protection and lubricant containment than shields, but have lower maximum speed capabilities. Open bearings offer no protection from contamination or lubricant containment, but have higher maximum speed capabilities than bearings with seals or shields. 7

8 Self aligning Double Row Deep Groove This bearing style is a modified version of the single row deep groove ball bearing through the addition of a second raw of balls. This design enlarges its capability to carry radial and thrust loads, due to the increased number of balls. Though using the same radial mounting space as the single-row style, its increased width lowers the capability of this bearing to accommodate misalignment 8

9 Manufacturing of Deep Groove bearings Video 1 Advantages of Rolling Bearing Low starting moment. Low friction at all speeds. Low energy consumption. High reliability. Small width. Low consumption of lubricant. Long re-lubrication intervals. Easy to mount and dismount. Standardized dimensions 9

10 11-2 Bearing Life If a bearing is clean, properly lubricated and mounted and is operating at reasonable temp., failure is due to fatigue caused by repeated contact stresses (Hertzian stress) Fatigue failure consists of a spalling or pitting of the curved surfaces Spalling crack initiates below the curved surface at the location of maximum shear stress, propagates to the surface causing surface damage. Failure criterion spalling or pitting of an area of 0.01 in 2, Timken company (tapered bearings) Bearing Life Fatigue failed Lycoming 320 main bearing. Contact pressure from the crankshaft seems to be more concentrated Main bearing wear. Contact occurs only on upper half of bearing. Crankshaft is not running true in bearing. Either crankshaft is bent or crankcase journals are mis-aligned 10

11 11-3 Bearing load Life Life number of revolution or hours of operation, at constant speed, required for the failure criterion to develop. Rating Life defines the number of revolution or hours of operation, at constant speed, in such a way that 90% of the bearings tested (from the same group) will complete or exceed before the first evidence of failure develops. This is known as life. For ball bearings and spherical bearings; L 10 = 500 (hours) x (rpm) x 60 = 10 6 = 1 million revolutions For tapered bearings manufactured by Timken: L 10 = 3000 (hours) x 500 (rpm) x 60 = 90 x 10 6 = 90 million revolutions Basic Dynamic Load Rating, C constant radial load that a group of bearings can carry for L 10 life. L Bearing load Life For a bearing with 90% reliability and given load and life, we can write a regression equation in the form / = (As per the ISO 281: standard) Where F is the applied radial load, a = 3 for ball bearings and a = 10/3 for roller bearings We can write / = / / = / = ( L 60 L 60 ) / F is in kn, L is in hours, n is in rpm 11

12 Example 11.1 Select a deep groove ball bearing for a desired life of 5000 hours at 1725 rpm with 90% reliability. The bearing radial load is 2kN. = 16.1 Read topics 11-4 & 11-5 These topics are important if reliability is not equal to 90%. Weibull distribution is used to determine relationship between the load, life, and reliability. = x o, ( -x o ) and b are Weibull parameters. R D is the desired reliability a f is the application factor serves as fos. 12

13 Example Combined radial and thrust loading Ball bearings can take both radial and thrust loading Both load can be expressed as F e = equivalent radial load that does the same damage as the combined radial and thrust loads V=1 when inner ring rotates and 1.2 when outer ring rotates (Rotation factor). X and Y are radial and thrust factors respectively (given) 13

14 11-6 Combined radial and thrust loading i = a factor whose value is 1 if and 2 if > Table 11-1 gives values of X 1, Y 1, X 2 and Y 2. e = minimum ratio b/w axial and radial loading bellow which axial load can be ignored e is a function of (Table 11-1) C 0 is the basic static load rating is the load that will produce a total permanent deformation in the raceway and rolling element at any contact point of times the diameter of the rolling element and is given by the manufacturer (Table 11-2) Combined radial and thrust loading = 1 when = when > Table

15 Example 11-4 An SKF 6210 angular contact ball bearing has an axial load F a of 1.8 kn and a radial load F r of 2.2 kn applied with the outer ring stationary. The basic static load rating C 0 is 19.8 kn and the basic load rating is 35 kn. Estimate the L10 life at a speed of 720 rpm Selection of Ball and Cylindrical Bearings Example 11-7 (No thrust) Shown in Figure is a gear-driven squeeze roll that mates with an idler roll. The roll is designed to exert a normal force of 5.25 N/mm of roll length and a pull of 4.2 N/mm on the material being processed. The roll speed is 300rpm, and a design life of 30kh is desired. Use an application factor of 1.2 and select a pair of angular-contact O2-series ball bearings from Table 11-2 to be mounted at O and A. Use the same size bearings at both locations with 90% reliability. 15

16 11-8 Selection of Ball and Cylindrical Bearings Problem (with thrust load) An 02-series single-row deep-groove ball bearing is to be selected from Table 11 2 for the application conditions specified bellow. F r F a Life Ring rotating Reliability 8 kn 2 kn 10 kh Inner, 400 rpm 99% Assume Table 11 1 is applicable if needed. Specify the smallest bore size from Table 11 2 that can satisfy these conditions. Solution: Since bearing is not known, e can not be determined to check weather i =1 or 2. Start analysis from the middle of Table 11-1 with X = 0.56, Y = 1.63 Iterate until getting the result Selection of Tapered Roller Bearings 4 parts 1. Cup 2. Cone 3. Tapered rollers 4. Cage Two separable parts G is the location of the effective load center. Use this point to estimate radial bearing load. 16

17 11-9 Selection of Tapered Roller Bearings Can carry both radial and axial load, or any combination Even if no axial load, due to configuration will have thrust reaction. DIRECT or INDIRECT mounting arrangements can be used to eliminate this reaction force (section for mounting details) A o and B o are the shaft reaction locations For the shaft as a beam, a e is the effective spread a g is greater for direct mounting than indirect 11-9 Selection of Tapered Roller Bearings If a e is same, the bearing are closer in Indirect mounting Due geometric configuration, even radial load will produce thrust reaction F i which is (by Timken) K is the ratio of the radial load rating to the thrust load rating As first estimate, K = 1.5 for radial and 0.75 for steep angle bearings Force vectors in Direct mounting 17

18 11-9 Selection of Tapered Roller Bearings Since bearings experience both axial and radial forces, then; Timken suggests (for all cases) F r is the radial load and F a is the net axial load induced by the other pair bearing and external load Only one bearing will carry the net axial load, which one? depends on Mounting direction External load direction Weather Shaft or housing is moving 11-9 Selection of Tapered Roller Bearings Selection methodology 1. Visually determine the bearing which is squeezed by the external load F ae and label it A, other is B 2. Determine which bearing carries the net axial load. Generally A will carry. 3. If the induced thrust F ia from bearing A happens to be larger than the combination of the external thrust and the thrust induced by bearing B, then bearing B will carry the net thrust load. 18

19 11-9 Selection of Tapered Roller Bearings Selection methodology 4. Timken recommends 5. Once the equivalent radial loads are determined, they should be used to find the catalog rating load using any of Eqns. discussed earlier. 6. Timken uses 2 parameters Weibell parameters with x o = 0 for Reliability other than Selection of Tapered Roller Bearings Bearing Reliability Bearing Reliability greater than 0.90 is given by; 19

20 Example 11-8 The shaft depicted in Fig. carries a helical gear with a tangential force of 3980N, a radial force of 1770N, and a thrust force of 1690N at the pitch cylinder with directions shown. The pitch diameter of the gear is 200mm. The shaft runs at a speed of 800rpm, and the span (effective spread) between the direct-mount bearings is 150mm. The design life is to be 5000h and an application factor of 1 is appropriate. If the reliability of the bearing set is to be 0.99, select suitable single-row tapered roller Timken bearings. Example

21 Example Consider a constrained housing as depicted in Fig. with two direct-mount tapered roller bearings resisting an external thrust F ae of 8000 N. The shaft speed is 950 rev/min, the desired life is h, the expected shaft diameter is approximately 1 in. The reliability goal is The application factor is appropriately a f = 1. (a) Choose a suitable tapered roller bearing for A. (b) Choose a suitable tapered roller bearing for B. (c) Find the reliabilities R A, R B, and R. Solution Reactions Bearing B is unloaded, R = R A = 0.95 With no radial load, F i B = 0, and using K = 1, we get Use K = 1 to get F ea = F ae = 8000 N and F e B = F r B = 0, also Use equation to calculate C 10 21

22 Solution a) Select cup HM88610 and cone HM88630 with thrust load of N b) Bearing B has no load so use a simple ball or roller bearing The actual reliability of bearing A will be The actual reliability of Bearing B will be (F D = 0) c) The combined reliability is Which is greater than 0.95 and is OK Installation of Bearing Mounting and enclosures 22

23 11-12 Mounting and enclosures Bearing bracket Bearing Housings Mounting and enclosures Tapered bearing in a car front wheel Mounting for a washingmachine spindle (Indirect mount) 23

24 Mounting of Angular Ball Bearing When face-to-face (DF) duplex pairs are mounted, the inner rings abut and the outer rings are drawn together, providing a higher radial and axial stiffness and accommodation of misalignment. Mounting of Angular Ball Bearing When a back-to-back (DB) duplex pair is mounted, the outer rings abut and the inner rings are drawn together, providing maximum stiffness. 24

25 Mounting of Angular Ball Bearing With tandem (DT) pairs, both inner and outer rings abut and are capable of sharing a thrust load, providing increased thrust capacity. Mounting of Angular Ball Bearing DF Face-to-face; will take heavy loads and thrust leading from either direction. DB Back-to-back; greatest aligning stiffness and is also good for heavy radial loads and thrust loads from either direction. DT Tandem arrangement; is used where thrust is always in the same direction. Show video 2 25

26 Alignment As general, the permissible misalignment is to radians for deep-groove ball bearings for spherical ball bearings for tapered roller bearings The life decreases by 20% for every radians after the given limit. Sealing for Enclosures and Bearings Read 26

27 Problems Problems: 11-1, 11-2, 11-3, 11-7, 11-8, 11-10, , 11-20, 11-21, 11-22, 11-24, 11-25, 11-31, 40, 43 27